Since the diameter of the light-receiving or input end of a fiber is usually much smaller than that of the output end of a laser cavity, it is customary to interpose a converging lens between the laser and the fiber for focusing the laser beam upon the fiber axis. Thus, for example, a gas laser with a power of about 10 to 100 W may have a cavity 1 to 10 mm in diameter compared with a fiber diameter of 0.3 mm. The lens should have a fairly large focal length so that the angle of incidence of the limiting rays of the converging beam does not exceed the critical angle of the fiber material.
For a proper focusing of the laser beam upon the fiber axis, the lens should have a mounting enabling its translational as well as angular adjustment relative to the laser and the fiber. Such mountings have a rather complex structure and are correspondingly expensive to produce.